THE PLT RPRTD HE HAD MADE SEVERAL GLIDER TOW FLTS FROM THE GRASS STRIP, WHICH INITIALLY WAS FROZEN. ON THE LAST 2 FLTS, HE TOOK OFF TO THE WEST & LANDED TO THE EAST IN PREPARATION FOR THE NEXT TOW. AFTER THE 7TH TOW, HE HAD PLANNED A 'LONG' LANDING & THE ACFT TOUCHED DOWN PAST THE HALFWAY POINT IN CALM WIND. THE PLT RPRTD THAT WHEN HE APPLIED BRAKES, BRAKING WAS INEFFECTIVE. THE ACFT CONTD OFF THE END OF THE RWY & WENT THRU 2 FENCES BEFORE STOPPING. AT THE TIME OF THE ACDNT, THE FROZEN GROUND HAD JUST STARTED TO THAW AT THE SURFACE WHICH MADE THE GRASS SLIPPERY.